1. Field of the Invention
The present invention relates to a method of purifying an alkaline solution by effectively non-ionizing (neutralizing) or removing metallic impurity ions therein and a method of etching semiconductor wafers with a purified alkaline solution without deteriorating the quality thereof.
2. Description of the Related Art
Generally, the manufacturing method of semiconductor wafers includes several steps: a slicing step to obtain wafers of thin disc shape by slicing a single crystal ingot formed by pulling the crystal in a crystal pulling machine; a chamfering step to chamfer a peripheral edge portion of the wafer obtained by the previous step to prevent cracking or breakage of the wafer; a lapping step to flatten the surface of the chamfered wafer by lapping it; an etching step to remove processing damage of the chamfered and lapped wafer; a polishing process to polish the surface of the etched wafer; and a cleaning step for cleaning the polished wafer to remove the polishing agent or dust particles from its surface.
There are two methods to etch a silicon wafer namely, an acid etching method using an acid etching solution of a mixed acid or the like and an alkaline etching method using an alkaline etching solution of NaOH or the like. In the acid etching, the etching speed is fast and thus it is difficult to uniformly etch the wafer without deteriorating the flatness of the wafer. Therefore, alkaline etching which uses a sodium hydroxide solution, a potassium hydroxide solution and an alkyl ammonium hydroxide solution etc. is predominantly employed because alkaline etching does not deteriorate the flatness of the wafer due to its uniform etching even if its etching speed is slower.
In the alkaline etching of the semiconductor wafer, an available industrial alkaline solution of high metallic impurity concentration is typically used as it is. A lot of metallic impurities are included in the industrial grade alkaline solution which is generally used. Even an electronic industrial grade alkaline solution used for etching semiconductor wafers contains metallic impurities of several tens of parts per billion (ppb) to several parts per million (ppm).
There are nickel, chromium, iron and copper metallic impurities included in this alkaline solution. A lot of nickel, chromium and iron, which are raw materials of stainless steels used in the manufacturing process of the alkaline solution, are especially included.
Conventionally, it was thought that these metallic impurities contaminate only the surface of the semiconductor wafer when the wafer was etched with an alkaline solution which contained these metallic impurities. It was assumed that metallic impurities, which adhere to the surface of the wafer, are removed enough by cleaning the wafer with an acid solution. Accordingly, it was thought that the existence of metallic impurities in the alkaline etching solution did not affect the wafer quality.
The present inventors researched the alkaline etching process over many years. Recently the inventors discovered, contrary to traditional understanding, that metallic ions of the metallic impurities such as copper and nickel, which exist in the alkaline etching solution, diffuse deep inside of the wafer while etching. The result of this phenomena is that the wafer quality is deteriorated and the resulting semiconductor device made of the wafer is remarkably degraded.
One might think to use the alkaline solution of high purity to prevent deterioration in the wafer quality due to the alkaline etching solution. However, an alkaline solution of high purity is the extremely expensive analysis grade alkaline solution. It does not make economical sense to employ an analysis grade alkaline solution for industrial use.